Several general developmental problems are under study. I. One particular problem is an analysis of the in vitro growth of intact mandibular arches from young mouse embryos. Technical procedures are varied in an effort to obtain a system which simultaneously permits cohesive development of the arch and differentiation of internal tissues. II. The capabilities for differentiation of arch tissues is also studied by growth of isolated tissues from embryos of different ages. It is the goal of these studies to determine the embryonic age at which the embryonic arch tissues are determined to differentiate. III. Homologous and heterologous tissue recombinations are planned so that the range of capabilities for differntiation of arch tissues may be determined. IV. Also planned are scanning electron microscope studies of the surface of mandibular arches from embryos of different ages and SEM observations of the internal tissues from embryos of selected ages.